An electric car that is a power car for a railway vehicle has a configuration in which power is obtained by a current collector from an overhead wire, a third rail, or the like (hereinafter, abbreviated as “overhead wire or the like” as needed) to drive an electric motor that uses the collected power. In the electric car, a DC/DC conversion device is used as a functional unit that converts a DC voltage applied from the overhead wire or the like to a different DC voltage (a lower voltage level for the electric car).
The DC/DC conversion device is configured to include at least one voltage-conversion circuit unit (which is also referred to as “power-conversion circuit unit”) in which an upper-arm switching element and a lower-arm switching element are connected in series. While it is more common to use a power-conversion circuit unit with a two-level configuration, a power-conversion circuit unit with a three-level configuration, in which the potential at a neutral point can be used, is used in a case where the input voltage is higher relative to the withstand voltage of the switching elements. In a power-conversion circuit unit with a three-level configuration, two capacitors are provided on the input-terminal side thereof, with the capacitors being connected in series and being equal in capacitance value. This is so that it is possible to select from among three potential levels: a high potential, an intermediate potential, and zero potential.
In a power-conversion circuit unit with a three-level configuration, a capacitor on the high-potential (high-order potential) side and a capacitor on the low-potential (low-order potential) side are equal in capacitance value. Therefore, the voltages of both the capacitors are typically equal to each other. However, it is known that, depending on the control mode of the switching elements in the power-conversion circuit unit, differences occur in voltage (an imbalance) between the capacitors. When there is a difference in voltage between the capacitor on the high-potential side and the capacitor on the low-potential side, the potential at the neutral point varies, which is not desirable for the operation of a power-conversion circuit unit.
For the DC/DC conversion device with a three-level power conversion unit as described above, there is an example disclosed in Patent Literature 1 listed below of a technique using a pulse-width modulation device in order to reduce variations in potential at a neutral point. With the pulse-width modulation device, according to the magnitude of a DC component included in each of two switching functions for respectively generating a high-potential pulse train and a low-potential pulse train of an output terminal voltage, or according to the value equivalent to the magnitude of the DC component, a difference in the DC component between the switching functions is adjusted; or according to the voltage difference between two divided voltages of a DC voltage source at a DC-side intermediate potential point, or according to the value equivalent to the voltage difference, the difference in an AC component between the switching functions is adjusted. Thus, variations in potential at a neutral point are thereby reduced.